The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density for use as a power source in these portable electronic instruments. Such a lithium secondary battery, having an average discharge potential of 3.7 V (i.e., a battery having substantially a 4 V average discharge potential) is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as cellular telephones, notebook computers, camcorders, etc. (i.e., the “3C” devices).
Also, there has been extensive research on batteries with effective safety characteristics such as preventing overcharge. When a battery is overcharged, an excess of lithium ions is deposited on a positive electrode, and an excess of lithium ions is also inserted into a negative electrode to make the positive and negative electrodes thermally unstable. An eruptive explosion occurs from a decomposition of the electrolytic organic solvent, and the thermal runaway that occurs causes serious problems of battery safety.
To overcome the above problems, it has been suggested that an aromatic compound such as an oxidation-reduction additive agent (“redox shuttle”) be added to the electrolyte. For example, U.S. Pat. No. 5,709,968 discloses a non-aqueous lithium ion secondary battery to prevent thermal runaway resulting from overcharge current by using a benzene compound such as 2,4-difluoroanisole. U.S. Pat. No. 5,879,834 discloses a method for improving battery safety by using a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene, furan, etc. which is polymerized electrochemically to increase the internal resistance of a battery during unusual overvoltage conditions. Such redox shuttle additives increase the temperature inside the battery early due to heat produced by the oxidation-reduction reaction, and close pores of a separator through quick and uniform fusion of the separator to inhibit an overcharge reaction. The polymerization reaction of these redox shuttle additives consumes the overcharge current to improve battery safety.
However, the polymerization of these redox shuttle additives cannot sufficiently eliminate the overcharge current. In addition, decomposition of the additives causes gas generation inside the battery, and thus, a certain plane of the battery swells. Therefore, improvements in the safety of the battery are limited when using the redox shuttle additives. Additionally, some redox shuttle additives have a deleterious effect on electrochemical properties such as high temperature or cycle life characteristics.
For solving the swelling phenomenon from gas generation inside the battery, a method has been disclosed in which the safety of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting an internal electrolyte solution when the internal pressure is increased above a predetermined level. However, a problem with the disclosed method is that the battery may operate incorrectly because of an increase in internal pressure itself.